For a wind turbine to optimise its energy output, the wind turbine must be controlled to face directly into the wind. Usually, a wind turbine comprise a number of rotor blades mounted to a spinner at the front of a nacelle, and the nacelle is mounted on top of a tower and can be rotated or “yawed” by a yaw drive. In most prior art wind turbines, the momentary wind direction is estimated using measurement input provided by a weather station mounted on the nacelle. For example, a wind vane is usually used to estimate the wind direction, and this information is translated into yaw drive control commands. However, information from such measurement systems is inaccurate since turbulence caused by the rotor blades means that the data collected by the measurement systems does not reflect the actual situation at the front of the wind turbine or in the rotor plane, i.e. the plane through which the rotor blades move. As a result of these inaccuracies, the nacelle may be turned at a degree of “yaw error”, i.e. so that the nacelle, spinner and rotor plane are not facing directly into the wind. The reduction in turbine efficiency resulting from even minor degrees of yaw error can accumulate to result in a significant reduction in annual energy production (AEP). Furthermore, the uneven loading that results from the skewed position of the nacelle, spinner and rotor plane relative to the wind can also significantly increase fatigue loading.